Under the proposed changes judges can impose shorter or longer sentences than the baseline according to the severity of the crime, but must also fix a mandatory non-parole sentence as a proportion of the total sentence.

The proportion will be at least 60 per cent of the total sentence for offenders sentenced to fewer than 20 years in jail and at least 70 per cent for criminals sentenced to 20 years or more in jail.

Offenders sentenced to life in prison must serve at least 30 years.

The legislation would double the average sentence for commercial drug trafficking, which is seven years at present.

Advertisement

Mr Clark says the laws, which go before parliament on Thursday, mean courts must impose longer sentences for these crimes across the sentencing range with the aim of dramatically increasing the average sentence to:

25 years for murder;

14 years for large-scale commercial drug trafficking, up from seven years;

10 years for incest with a minor;

10 years for sexual penetration with a child under 12, up from 3.5 years;